I just went through something similar with my cooling mist pump.
I thought it was a kink in the line or air somewhere, or debris. Checked everything over 2 months time and could not get it to work. Everyone told me the pump was unlikely the culprit.
I met a local guy swapped pumps and BAM, full meth flow again. So I bought a new pump and all is good.
I was going to pull my pump apart to see if I could identify what made it go weak.